Coelenteramide (CTMD) is an oxidation product of coelenterazine. CTMD is produced in the luminescence process of the calcium-binding photoproteins from coelenterates and in the luminescence process of coelenterazine (CTZ) type luciferases from Renilla, Gaussia, etc.

The calcium-binding photoprotein is a member of the photoproteins which emits light upon specific binding with Ca2+ and is a complex of the peroxide of coelenterazine and an apoprotein (Non-Patent Literature 1. Head, J. F. et al. (2000) Nature 405: 372-376). Among the calcium-binding photoproteins, a representative photoprotein is aequorin, which is derived from luminous Aequorea victoria. In addition to aequorin, obelin, mitrocomin, clytin-I, clytin-II, etc. are known. Their luminescence mechanisms are considered to be basically the same. After the luminescence reaction of aequorin by the addition of Ca2+, the aequorin solution shows blue fluorescence. It is therefore considered that a complex of CTMD which is the oxidation product of CTZ and apoaequorin-Ca2+ would be formed, and this complex is referred to as BFP (Blue Fluorescent Protein). In recent years, the process for the quantitative preparation of BFP from recombinant aequorin has been established (Non-Patent Literatures 2 and 3: Inouye, S. (2004) FEBS Lett. 577:105-110, Inouye, S, and Sasaki, S (2006) FEBS Lett. 580: 1977-1982). It has been shown that BFP is a fluorescent protein with heat stability and has a luciferase activity when CTZ serves as a substrate. On the other hand, it was also shown that a greenish fluorescent protein (gFP) is produced by removing Ca2+ from apoaequorin-Ca2+. Furthermore, the method for regenerating aequorin having luminescence activity has been also established to form gFP by adding CTZ. BFP is a only protein that has the fluorescence ability and luciferase activity and can be regenerated to aequorin. Accordingly, BFP has possibilities to apply as a reporter protein in the field of cell biology and is remarkable protein.
The following fluorescent coelenteramide-related compounds are disclosed in Non-Patent Literatures 4 to 21.

The structures of the coelenteramide-related compounds described above and their fluorescence properties are described in Non-Patent Literatures 4 to 21.